1. Field of the Invention
In the case of forming a semiconductor layer on a substrate through the use of what is called the CVD (Chemical Vapor Deposition) method including vapor phase epitaxy, or plasma CVD method using electric or photo energy, reactive gases, such as a reactive semiconductor material gas which is the raw material of a semiconductor forming the semiconductor layer and a reactive impurity material gas which is the raw material of an impurity imparting a conductivity type to the semiconductor layer.
The present invention relates to a method of purifying reactive gases which are employed for the fabrication of a semiconductor layer through the CVD method.
2. Description of the Prior Art
The reactive gas for use in the formation of a semiconductor layer on a substrate by the CVD method is widely on sale.
The reactive gas now placed on the market is usually loaded into a container made of iron or what is called a Bombe.
The reactive gas before being loaded into the Bombe has as high a purity as 6 nines (99.9999) to 7 nines (99.99999) and contains unwanted impurities no more than 1 ppm.
However, the reactive gas commercially available at present usually contains unnecessary impurities, such as oxygen, carbon, silicon oxide, hydrocarbon, heavy metal and so forth, in as large quantities as 0.01 to 0.1% on the ground that when the reactive gas is packed into the Bombe, the surrounding gas (usually air) enters into the Bombe.
With such a reactive gas containing large quantities of undesirable impurities, it is impossible to form a semiconductor layer with required electric and photoelectric characteristics.
For example, in the case of forming a non-single-crystal semiconductor layer through using a reactive semiconductor material gas, if the reactive gas contains oxygen, the resulting non-single-crystal semiconductor layer would contain oxygen clusters which act as recombination centers. And, in this case, if the non-single-crystal semiconductor layer has dangling bonds, then it would contain oxygen which acts as a doner center making the layer N type. Furthermore, if the reactive semiconductor material gas contains oxygen, then the semiconductor forming the non-single-crystal semiconductor layer would be coupled with the oxygen to form a semiconductor oxide which acts as a barrier against electrons or holes or as an insulator.
As described above, in the case of forming the non-single-crystal semiconductor layer throught using the reactive semiconductor material gas, if the reactive gas contains oxygen, then required electric and photoelectric characteristics of the semiconductor layer cannot be obtained.
Furthermore, for instance, in the case of forming a non-single-crystal semiconductor layer containing microcrystals and having regularity of such a short range order between 5 to 200 .ANG. through using the reactive semiconductor material gas, if the reactive gas contains oxygen, then it would be impossible to form such a non-single-crystal semiconductor layer desired to obtain. In consequence, the non-single-crystal semiconductor layer cannot be formed with required electric and photoelectric characteristics.